Acoustic wave resonator and method of operating the same to maintain resonance when subjected to temperature variations

ABSTRACT

An acoustic resonator includes a ferromagnetic compensator which at least partially offsets temperature-induced effects introduced by an electrode-piezoelectric stack. The compensator has a positive temperature coefficient of frequency, while the stack has a negative temperature coefficient of frequency. By properly selecting the thickness of the compensator, temperature-induced effects on resonance may be neutralized. Alternatively, the thickness can be selected to provide a target positive or negative composite temperature coefficient of frequency. In the preferred embodiment, the compensator is formed of a nickel-iron alloy, with the most preferred embodiment being one in which the alloy is approximately 35% nickel and approximately 65% iron. In order to prevent undue electromagnetic losses in the ferromagnetic compensator, a metallic flashing layer may be added to at least partially enclose the compensator.

TECHNICAL FIELD

[0001] The invention relates generally to acoustic resonators and moreparticularly to approaches for controlling the resonant frequency of thesame.

BACKGROUND ART

[0002] Acoustic resonators that are formed of thin films may be used ina number of applications that require a precisely controlled frequency.A Thin Film Bulk Acoustic Resonator (FBAR) or a Stacked Thin Film BulkAcoustic Resonator (SBAR) may be used as a filter in a cellulartelephone or other device in which size, cost and frequency stabilityare important factors.

[0003] An FBAR includes a thin film of piezoelectric material betweentwo conductive electrodes, while an SBAR includes additional layers ofpiezoelectric material, with each such layer separating two electrodes.While solidly mounted resonators are known, the active layers of an FBARor SBAR are often suspended in air by supporting the layers around theperimeter. The air/resonator interfaces at both sides of the stack oflayers partially trap the energy that is generated during operation.

[0004] When a time-varying electrical field is created by applying asignal across two electrodes that are separated by a piezoelectriclayer, the piezoelectric material converts some of the electrical energyinto mechanical energy in the form of sound waves. The sound wavespropagate in the same direction as the electrical field and arereflected at the air/resonator interfaces. For a properly fabricatedFBAR or SBAR, the sound waves will have a particular mechanicalresonance.

[0005] As mentioned above, an FBAR or SBAR can be used as a filter,since it will function as an electronic resonator when allowed tooperate at its mechanical resonant frequency. At this mechanicalresonant frequency, the half wavelength of the sound waves propagatingthrough the resonator is approximately equal to the total thickness ofthe resonator for a given phase velocity of sound in the piezoelectricmaterial. Acoustic resonators may be used alone or in combination. Forexample, a bandpass filter is formed by electrically connecting severalresonators to provide a desired filter response. Several filtertopologies are possible. One favored topology is the half-laddertopology, where a group of resonators are connected in series (seriesresonators) and in between the series resonators are shunt resonatorsthat are connected to ground. The series resonators are fabricated suchthat their resonant frequency is approximately 3% higher than the shuntresonators. Since the thickness of the piezoelectric layer can be thesame for the series and shunt resonators, the piezoelectric depositionis often “shared” between resonators.

[0006] It becomes manifest that an important characteristic of acousticresonators is an ability to maintain resonance. This has provedproblematic when acoustic resonators are placed in an environment thatundergoes temperature fluctuations, since a frequency shift (Δf) willoccur if a variation in temperature (ΔT) induces a change in thethickness (Δt) and/or wave velocity (ΔV) for one or more layers of aresonator. Specifically, the resonant frequency f₀ and the temperaturecoefficient of frequency are respectively defined as follows:

f ₀ =V/2t ₀  (1)

Δf/f ₀ =ΔV/V−Δt/t ₀  (2)

[0007] where V is the velocity of the acoustic wave propagating throughthe acoustic resonator and t₀ is the thickness of the resonator. Thethickness is defined in terms of the acoustic wavelength as follows:

t ₀=λ/2  (3)

[0008] where λ is the wavelength of the acoustic wave in the mediumthrough which it propagates. In the materials employed to fabricateacoustic resonators, the thickness, t₀, usually increases with positivechanges in temperature, ΔT. On the other hand, the velocity of wavepropagation through the materials usually decreases with positivechanges in temperature. These two factors combine to provide thephenomenon that is referred to as negative temperature coefficient offrequency. From equations (1) and (2) it is seen that the resonantfrequency, f₀, of an acoustic resonator generally decreases as thetemperature increases. This fluctuation in resonance is often anundesirable characteristic.

[0009] One known approach to compensating for the variations intemperature is to incorporate a frequency stabilization circuit.However, space restrictions of cellular phones and similar devicesimpose limitations on the use of auxiliary circuits. Another approach isdescribed in a paper entitled “Thin Film Resonators and Filters” by K.M. Lakin, 1999 IEEE Ultrasonics Symposium, Jun. 1, 1999. This secondapproach applies to solidly mounted resonators (SMRs), which are mountedalong a supporting surface, rather than being suspended by peripheralsupport from the supporting surface. Acoustic isolation being an SMR andthe substrate in which it is formed is achieved by forming a reflector(typically a Bragg reflector) between the SMR and substrate. Thereflector is a layer stack having alternating layers of high index andlow index materials, with each layer having a thickness of approximatelyone-quarter wavelength of the resonant frequency of the SMR. Accordingto the second approach, if silicon dioxide (SiO₂) is used to form one ofthe index-specific layers, a degree of temperature compensation willoccur as a result of the temperature coefficient of SiO₂. However, adrawback is that SiO₂ is hydrophilic, so that the performance of the SMRmay degrade in a humid atmosphere. Another concern is that the level ofcompensation is partially determined by the target resonant frequency,since the SiO₂ is formed as one-quarter wavelength layers.

[0010] What is needed is an acoustic resonator and method of using thesame that maintains resonance when subjected to variations intemperature.

SUMMARY OF THE INVENTION

[0011] An acoustic resonator includes an electrode-piezoelectric layerstack having a negative temperature coefficient of frequency that is atleast partially offset by acoustically coupling a compensator to theelectrode-piezoelectric stack. The compensator is formed of a materialwith properties that cause the compensator to countertemperature-induced effects on resonance, where such effects areintroduced by temperature variations to the electrode-piezoelectricstack.

[0012] In one embodiment, the compensator is formed of a ferromagneticmaterial. In the more preferred embodiment, the material is anickel-iron alloy, with the most preferred embodiment being one in whichthe alloy consists of approximately 35% nickel and approximately 65%iron. The compensator should exhibit a positive coefficient offrequency. The thickness of the compensator may be selected such thatthe magnitude of the temperature-induced effects on resonance, as aresult of the presence of the compensator, is substantially equal to themagnitude of the temperature-induced effects on resonance of thenegative temperature coefficient of frequency of theelectrode-piezoelectric stack. As one example, it is believed that a 0ppm/° C. composite coefficient can be obtained if the nickel-iron alloycompensator has a thickness of 3320 Å, while the stack includesmolybdenum electrodes having thicknesses of 1100 Å on opposite sides ofan aluminum nitride layer having a thickness of 15,200 Å.

[0013] Ferromagnetic materials have the disadvantage of being associatedwith large electrical losses at microwave frequencies. To prevent this,a flash layer of molybdenum may be used to encase the ferromagneticalloy and divert the current flow around it. For example, a thin layer(e.g., 200 Å) of molybdenum may be formed on a side of the compensatoropposite to the electrode-piezoelectric stack. While other materials maybe used, the preferred embodiment is one in which the flashing materialis the same as the electrode material.

[0014] Still referring to the preferred embodiment, the compensator andthe electrode-piezoelectric stack are suspended from the surface of asubstrate. Thus, it is not necessary to include a Bragg reflector orother mechanism for allowing the resonating layers to be in contact witha substrate.

[0015] One advantage to the invention, relative to prior means ofproviding compensation for the negative temperature coefficient offrequency typically exhibited by electrode-piezoelectric stacks, is thatthe compensator of the invention may have a thickness that isindependent of the desired wavelength of the target resonant frequency.That is, rather than having a thickness that is selected to be aone-quarter wavelength layer within a Bragg reflector, the thickness ofthe compensator may be selected to tailor the compensating capabilitiesof the compensator. Another advantage of the invention is that thecompensator is formed of a metal, so that the electrical resistance ofthe electrodes is not significantly affected. Yet another advantage isthat the preferred nickel-iron alloy can be etched using the same wetetch as is conventionally used to pattern the electrodes. Moreover, thecompensator is not hydrophilic, so that it does not degrade in a humidenvironment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a side sectional view of an acoustic resonator inaccordance with the present invention;

[0017]FIG. 2 is a side sectional view of the acoustic resonator, shownin FIG. 1, connected to a voltage source;

[0018]FIG. 3 is a side sectional view of the acoustic resonator, shownin FIG. 1, formed on a wafer in accordance with the present invention;and

[0019]FIG. 4 is a side sectional view of the acoustic resonator, shownin FIG. 3, in accordance with an alternative embodiment of the presentinvention.

DETAILED DESCRIPTION

[0020] Referring to FIG. 1, an exemplary embodiment of an acousticresonator 10 includes a pair of opposed electrodes 12 and 14. Betweenthe opposed electrodes 12 and 14 is a piezoelectric body 16, forming anelectrode-piezoelectric stack 18. The stack 18 is typically referred toas a thin film bulk acoustic resonator (FBAR).

[0021] Disposed adjacent to the stack 18 is a compensator body 20 thatfacilitates stabilization of the resonant frequency of the acousticresonator 10 when subjected to temperature variations. This is achievedby forming the compensator body 20 from one or more materials having apositive temperature coefficient of frequency. The positive temperaturecoefficient of frequency compensates for the negative temperaturecoefficient of frequency of the materials from which theelectrode-piezoelectric stack 18 is formed.

[0022] Typically, the piezoelectric body 18 is formed from anypiezoelectric material that provides a reasonably high electromechanicalcoupling constant and low dielectric constant. Examples of dielectricmaterials that may be employed are ZnO, AIN and lead zirconatetitantate. The electrodes 12 and 14 are constructed from a conductivematerial, such as aluminum, gold, molybdenum, titanium, tungsten and thelike. The characteristics of the materials from which the piezoelectricbody 18 and the electrodes 12 and 14 are formed vary in response tochanges in temperature. This results in the resonant frequency of theresonator 10 decreasing with increases in the temperature. Specifically,the velocity characteristics of these materials decrease with increasingtemperatures, so that there is a decreased velocity of an acoustic wavepropagating through the stack. In addition, the thickness of thematerials increases with increasing temperature.

[0023] It was found that the dominant contribution to temperaturechanges in resonance is attributable to changes in the velocitycharacteristics of the materials that form the stack. As a result, thecompensator body 20 is selected to have velocity characteristics thatchange positively with changes in temperature. To that end, thecompensator body 20 is formed preferably from a metallic alloyconsisting essentially of approximately 35% nickel and 65% iron. Thealloy is sold by International Nickel Company under the trademark INVAR.This alloy forms a layer having a thickness that is substantiallyinvariant to changes in temperature. The velocity characteristics,however, vary positively with changes in temperature. In a paperentitled “Zero Sound Anomaly in a Ferromagnetic INVAR Alloy,” by Y.Endoh et al. (Journal of the Physical Society of Japan, Vol. 46, No. 3,March 1979, pages 806-814), data was presented which suggests that INVARhas a temperature coefficient of frequency of +170 ppm/° C. As acomparison, a temperature coefficient of frequency for INVAR issuggested to be approximately +239 ppm/° C. in data presented by L I.Mañosa et al. in a paper entitled “Acoustic-mode VibrationalAnharmonicity Related to the Anomalous Thermal Expansion of INVAR IronAlloys,” Physical Review B, The American Physical Society, Volume 45,No. 5, Feb. 1, 1992, pages 2224-2236. By properly selecting the thickness of the compensator body 20, a resonator 10 may be formed having acomposite temperature coefficient of frequency that is substantiallyzero. Alternatively, the magnitude of the composite temperaturecoefficient of frequency may be set to a positive or negative value, sothat changes in resonance are proportional or negatively proportional tothe changes in temperature. In this manner, the resonator 10 may beprovided with virtually any thermal coefficient of frequency desired,dependent upon the application.

[0024] Referring to FIG. 2, a voltage source 22 is connected between theelectrodes 12 and 14. The electric field produced between the electrodes12 and 14 by the voltage source 22 generates, within the piezoelectricbody 16, an acoustic wave 24. The acoustic wave propagates between theplanes 26 and 28. The plane 26 represents the interface of thecompensator body 20 and surrounding ambient. The plane 28 represents theinterface of the electrode 14 and the surrounding ambient.

[0025] As previously noted, each of the layers of an acoustic resonatorhas a thickness (t) and a wave velocity (V) that vary with temperature.It is also known that the phase (φ) of waves propagating within theacoustic resonator between planes 26 and 28 will vary as a function ofthe frequency of the acoustic resonator. Specifically, the phase of thewaves propagating within the resonator 10 of FIGS. 1 and 2 is:

φ=2πf(t ₁ /V ₁ +t ₂ /V ₂ +t ₃ /V ₃ +t ₄ /V ₄)  (4)

[0026] where t₁ and V₁ relate to the properties of the bottom electrode14, t₂ and V₂ relate to the properties of the piezoelectric body 16, t₃and V₃ relate to the properties of the top electrode 12, and t₄ and V₄relate to the properties of the compensator body 20. Frequency stabilityis achieved when φ=π and the sum of the factors within the parenthesesof equation (4) is fixed. In the preferred embodiment of the invention,this sum is substantially fixed by allowing the fourth factor (i.e.,t₄/V₄) to vary inversely with the sum of the other three factors (i.e.,t₁/V₁+t₂/V₂+t₃/V₃).

[0027] As the temperature of the resonator 10 increases, the velocity V₂of the acoustic wave 24 in the piezoelectric body 16 decreases and thethickness t₂ increases. In addition, the velocity of the acoustic waveas it propagates through the electrodes 12 and 14, shown as V₁ and V₃,respectively, decreases and the thicknesses t₁ and t₃ increase. However,the decreased velocities of the acoustic wave are compensated by thepresence of the compensator body 20. Specifically, the velocity V₄ ofthe acoustic wave in the compensator body 20 increases with increasingtemperature. Were the magnitude of temperature-induced changes in thecompensator body equal to the magnitude of such changes in theelectrode-piezoelectric stack, there would be no appreciable change inthe overall phase φ of the acoustic waves propagating between planes 26and 28. Hence, the resonant frequency of the resonator 10 would bemaintained in the presence of temperature variations.

[0028] Referring to FIG. 3, in a specific example, a resonator 110 isfabricated employing conventional semiconductor processes. To that end,the resonator 110 is fabricated on a silicon wafer 130 having onesurface etched to form a cavity 132. This is typically achievedemploying KOH to remove a few microns of the silicon, referred to as apre-slotting. The amount of silicon removed by the KOH is chosen toavoid structural compromise of the wafer 130 during subsequentprocessing. Phospho-silicate glass 134 is deposited in the cavity and ispolished flat by known mechanical means. The deposition and subsequentremoval of the glass is described in U.S. Pat. No. 6,060,818 to Ruby etal., which is assigned to the assignee of the present invention.

[0029] After the glass 134 is formed in the cavity 132, one electrode114 is deposited on the surface of the wafer 130. In the presentexample, the electrode 114 is fabricated employing sputter depositiontechniques to form a layer of molybdenum approximately 1100 Å inthickness. This results in the electrode 114 having a negative thermalcoefficient of frequency of approximately −45 ppm/° C.

[0030] The piezoelectric body is formed by depositing a layer of AIN 116adjacent to the electrode 114 to have a thickness of approximately15,200 Å. This results in the piezoelectric body 116 having a negativethermal coefficient of frequency of approximately −25 ppm/° C.

[0031] The additional electrode 112, which is also formed frommolybdenum, is then deposited adjacent to the layer of AIN 116 to have athickness of approximately 1100 Å.The compensator body 120 is formedadjacent to the electrode 112 employing sputter deposition techniques tohave a thickness of approximately 3320 Å. The compensator body has apositive temperature coefficient of frequency of approximately, 170ppm/° C. During fabrication, the edges of the elements 112, 116 and 120are photolithographically etched to coincide with each other and withthe edges of the cavity 132. Thus, all points of the suspended stackwill have the same resonant frequency f₀. As a result, “spurious” nearbyresonances which would otherwise result from partial mass loadingeffects are prevented.

[0032] After deposition of the electrode 112 and compensator body 120,the phospho-silicate glass 134 is etched from under the resonatorstructure to complete the formation of the cavity 132. This may beachieved employing a dilute hydrofluoric acid solution. The removal ofthe remaining glass leaves a substantial portion of the electrode 114and, therefore, the resonator 110 spaced-apart from the wafer 130.

[0033] In this manner, a resonator 110 is formed having a thermalcoefficient of frequency of approximately 0 ppm/° C. Thus, fortemperature variations in the range of −30° C. to 85° C., the resonator110 may be fabricated to maintain a constant resonant frequency atapproximately 1.9 GHz in this example. Such techniques may be applied toform resonators at frequencies anywhere from 0.4 to 10 GHz.

[0034] Referring to FIG. 4, another embodiment of a resonator 210 isshown as including a flashing 230 covering the compensator body 220. Thepreferred materials from which the compensator body 220 is formed areferromagnetic materials, which may exhibit large electrical losses. Thepresence of this loss degrades the quality factor of the resonator, andis detrimental. The flashing 230 is included to provide a low losscurrent path around the compensator body 220. To that end, the flashing230 covers exposed regions of the compensator body 220, i.e, regionsthat are not positioned adjacent to the electrode 212. Although theflashing 230 may be formed from any conductive material, the flashing230 is preferably formed from the same materials as the electrodes 212and 214. The flashing is lithographically patterned so that the extramass of layer 230 on the edges of electrode 212 is over the siliconsubstrate 232. This will effectively damp any spurious resonances.

[0035] The flashing 230 and the electrodes 212 and 214 are formed of Moand each has a thickness of approximately 1100 Å. The piezoelectric body216 is formed from AIN having a negative thermal coefficient offrequency of approximately −25 ppm/° C. Therefore, were it desired tohave resonator 210 which exhibits a temperature coefficient of frequencyof approximately 0 ppm/° C., the compensator body 220 must have asufficient thickness to provide an offsetting positive temperaturecoefficient of frequency. With this configuration, the resonator 210 ofthe silicon substrate 232 would maintain a constant resonant frequencyfor resonators with frequencies chosen in the range of 200 MHz to 10GHz, and subjected to temperature variations in the range of −30° C. to85° C.

[0036] Various modifications to the present invention will becomeapparent to those skilled in the art from the foregoing description andaccompanying drawings. For example, the discussion has concerned an FBARtechnology. However, the invention is equally applicable to SBARtechnology. Moreover, the compensator bodies have been shown as being onthe side of the electrode-piezoelectric stack opposite to the substrate,but the compensator body could be formed directly on the substrate or ona flashing that contacts the substrate. Accordingly, the inventionshould not be limited to the exemplary embodiments discussed above, butshould be determined in view of the attached claims, including the fullscope of equivalents thereof.

What is claimed is:
 1. An acoustic resonator comprising: a substrate;and a layer stack integrated to said substrate such that said layerstack includes a suspended region, said suspended region including: apiezoelectric body and electrodes positioned to apply an electricalfield to said piezoelectric body, said piezoelectric body and electrodeshaving a resonance and a negative temperature coefficient of frequency;and a compensator acoustically coupled to said piezoelectric body andelectrodes, said compensator body being formed of a material havingproperties by which said compensator at least partially offsetstemperature-induced effects on said resonance, where saidtemperature-induced effects are a function of said negative temperaturecoefficient of frequency.
 2. The acoustic resonator of claim 1 whereinsaid compensator is a ferromagnetic layer that is spaced apart from saidpiezoelectric body by one of said electrodes, said ferromagnetic layerbeing associated with a positive temperature coefficient of frequency.3. The acoustic resonator of claim 1 wherein said layer stack includes aperipheral region that contacts said substrate to support said suspendedregion, said compensator being a layer of a nickel-iron alloy.
 4. Theacoustic resonator of claim 1 wherein said layer stack further includesa metallic flashing layer on a side of said compensator opposite to saidelectrodes and said piezoelectric body.
 5. The acoustic resonator ofclaim 1 wherein said layer stack is a thin film bulk resonator (FBAR)stack.
 6. The acoustic resonator of claim 1 wherein said compensator isformed of a material having a positive temperature coefficient offrequency and has a thickness such that a magnitude oftemperature-induced effects on said resonance by presence of saidcompensator is similar to a magnitude of said temperature-inducedeffects on said resonance as a function of said negative temperaturecoefficient of frequency.
 7. The acoustic resonator of claim 1 whereinsaid substrate is a silicon substrate and wherein said electrodes andcompensator are metallic layers.
 8. An acoustic resonator comprising: asubstrate; an electrode-piezoelectric stack having a target resonantfrequency and having a negative temperature coefficient of frequency;and a metallic compensator layer having a positive temperaturecoefficient of frequency, said metallic compensator layer beingacoustically coupled to said electrode-piezoelectric stack.
 9. Theacoustic resonator of claim 8 wherein said electrode-piezoelectric stackand said metallic compensator layer combine to define an FBAR.
 10. Theacoustic resonator of claim 9 wherein a major portion of said FBAR issuspended from contact with said substrate.
 11. The acoustic resonatorof claim 8 wherein said metallic compensator layer is formed of anickel-iron alloy.
 12. The acoustic resonator of claim 11 wherein saidnickel-iron alloy is approximately 35 percent nickel and approximately65 percent iron.
 13. The acoustic resonator of claim 8 wherein saidmetallic compensator layer has a thickness selected to neutralizeinfluences of temperature variations on resonance of saidelectrode-piezoelectric stack such that said target resonant frequencyis substantially maintained.
 14. A method of fabricating an acousticresonator comprising the steps of: providing a substrate; and forming amembrane on said substrate such that at least a portion of said membraneis suspended from contact with a substrate, including: (a) forming anelectrode-piezoelectric stack having a negative temperature coefficientof frequency, and (b) forming a compensator layer adjacent to saidelectrode-piezoelectric stack, including selecting a material having apositive temperature coefficient of frequency.
 15. The method of claim14 wherein said step (b) that includes selecting said material includesselecting a nickel-iron alloy.
 16. The method of claim 14 wherein saidstep (b) includes depositing said material as approximately 35 percentnickel and approximately 65 percent iron.
 17. The method of claim 14wherein said step (b) includes selecting a layer thickness tosubstantially match a magnitude of temperature-induced effects onresonance by operation of said electrode-piezoelectric stack with amagnitude of temperature-induced effects on said resonance as aconsequence of said compensator layer.
 18. The method of claim 14wherein said step of forming said membrane further includes (c) forminga metallic flashing layer on a side of said compensator layer oppositeto said electrode-piezoelectric stack.
 19. The method of claim 18further comprising using fabrication alignment techniques in said steps(b) and (c) to prevent spurious mode generation resulting from partialcoverage of said suspended membrane by said compensator layer or saidflashing layer.